In the case where the user wishes to iterate over all the documents and descend down all the subdocuments, $redact fails.

Consider the following syntax:

$redact: { $cond: [true, $$DESCEND, $$DESCEND] }

this will get serialized to:

$redact: $$DESCEND.

However, this fails with the error:

"exception: $redact specification must be an object".